ixp4xx dmabounce

[Brian Walsh]

On Thu, Sep 24, 2009 at 12:50 PM, Mikael Pettersson <mikpe at it.uu.se> wrote:
> Brian Walsh writes:
>  > On Wed, Sep 23, 2009 at 10:40 AM, Krzysztof Halasa <khc at pm.waw.pl> wrote:
>  > > Mikael Pettersson <mikpe at it.uu.se> writes:
>  > >
>  > >> I strongly suspect that something on the USB or networking side
>  > >> is allocating I/O buffers without observing the correct DMA APIs.
>  > >
>  > > At least the network stack allocates buffers ignoring the DMA masks.
>  > > The buffers may be allocated by one device (driver) and passed to
>  > > another device. The only plausible way to fix it is IMHO limiting all
>  > > skb allocations to the common mask (drivers would be free to either
>  > > handle or drop skbs outside of their mask).
>  > >
>  > > This is relatively easy to implement and I'm going to try it, when time
>  > > permits.
>  > >
>  > >> I think Krzysztof Halasa mentioned running ixp4xx devices with 128MB
>  > >> RAM and a kernel hacked so kernel-private allocations would always be
>  > >> served from memory below 64MB. I think he mentioned doing that because
>  > >> of networking components that would ignore PCI DMA mask constraints.
>  > >
>  > > Right. This works fine for network buffers because they aren't that
>  > > large. The current patch is suboptimal, though.
>  > > --
>  > > Krzysztof Halasa
>  > >
>  >
>  > I tried Krzysztof's patch and it had no noticeable affect.  I am still getting
>  > about 6.3 Mbps IP data throughput when only using the ohci controller and
>  > about 3.6 Mbps when the device is attached to the ehci controller.  This
>  > device works fine when running the same testing attached to an x86
>  > configured machine and gets about 18 Mbps IP data throughput.
>
> If your application can operate in 64MB RAM, you may want to try
> a kernel that includes only my ixp4xx disable dmabounce patch,
> and boot it with mem=64M. (Look in the kernel boot log and verify
> that it only sees 64M of RAM.)
>
> If performance increases, then your performance loss is due to bounces.
>

Mikael

I used your patch to disable legacy bounce, disabled support for > 64MB RAM,
and used the mem=64M kernel option.  There was no change in the data
throughput.

I am not sure where this leaves me.

Brian